Mechanism of contracture on cooling of caffeine‐treated frog skeletal muscle fibres.

1. In order to clarify the mechanism of contracture on cooling of caffeine‐treated intact muscle fibres, the temperature dependence of a calcium (Ca2+) release mechanism, 'Ca2+‐induced Ca2+ release', of the sarcoplasmic reticulum (SR) was examined in skinned frog muscle fibres. 2. Skinned fibres in a solution containing 1.2 mM‐caffeine and 0.7 mM‐EGTA (Mg2+, 1.5 mM, Mg‐ATP, 3.5 mM, pH 7), contracted on cooling (from 22 to 2 degrees C) due to Ca2+ release from the SR. 3. The rate of Ca2+ release from skinned fibre SR in a medium which contained Ca2+ ions (with 10 mM‐EGTA) and no ATP salts, was determined under various conditions using the 'caffeine method.' 4. In the absence of Mg2+ ions, adenine nucleotides and caffeine, the rates at room temperature (21‐22 degrees C) were 3‐4 times greater than those at a lower temperature (1.5‐3 degrees C), at any concentrations of Ca2+ ions external to the SR. 5. In the presence of Mg2+ ions (1.5 mM) and beta,gamma‐methylene ATP (1 mM), the effect of temperature on the rates disappeared in Ca2+‐containing media, although the effect remained in Ca2+‐free medium. 6. When caffeine (1.2 mM), which is a potentiator of the Ca2+‐induced Ca2+ release, was added to the test medium with Mg2+ and beta,gamma‐methylene ATP, the resulting potentiating effect was several times greater than that at lower temperature. 7. In order to examine the temperature dependence of the Ca2+ pump activity of the SR, the initial rate of Ca2+ uptake by the empty SR was determined under various conditions in the presence of Mg2+ ions (1.5 mM) and Mg‐ATP (3.5 mM). The Q10 of the pump activity was around 2.0 at the Ca2+ ion concentrations examined (less than 10(‐6) M). 8. A numerical model based on the results obtained, together with some reasonable assumptions, suggested that both suppression of the Ca2+ pump and enhancement of the Ca2+ release contribute to the cooling contracture of caffeinized fibres.